1. Field of the Invention
The present invention is in the field of hand trucks and in particular there is disclosed a power assisted hand truck for climbing steps which operates according to a new principle.
2. The Prior Art
U.S. Pat. No. 3,053,546 to Muirhead shows a hand truck equipped with a pneumatic jack for use in climbing stairs. The jack is initially vertical, and when activated it raises the hand truck vertically until a required height is reached. The extension is manually triggered and when the trigger is released the ram portion of the jack begins to retract. The ram is either extending or retracting at any particular time and is stationary only when fully extended or retracted. When the wheels of the hand truck are off the step, the entire load is balanced on the foot of the ram. When the wheels of the hand truck have been elevated sufficiently, they may be pulled by the operator onto the next step, the ram and its cylinder pivoting with respect to the frame of the hand truck. Thereafter, the ram is retracted and the cylinder is restored to its initial position by a spring.
In U.S. Pat. No. 3,417,831, Lake describes a hand truck having a pneumatically powered ram having a pneumatic cylinder which is pivotally attached to the frame of the hand truck and which swings with respect to it in a plane perpendicular to the axis of the wheels. The Lake hand truck has a single common axle extending the width of the truck, to which both wheels are mounted. The presence of this axle requires that the cylinder be mounted between the axle and the load, and this in turn limits the extent to which the cylinder can pivot.
The pneumatic valves in Lake's hand truck are electrically controlled and a wet battery is carried on the hand truck to operate them. Because of the presence of the wet battery, the hand truck cannot be laid down and, in addition, separate maintenance of a second power system in addition to the pneumatic power system is required. Also, the electrical switching represents an explosion hazard in certain environments.
In one embodiment, Lake shows a hand truck equipped with endless glide belts which bear against the edge of the step when the wheels are elevated. However, as seen from the side, the profile of the glide belt does not merge tangentially with the profile of the wheel, thereby causing a discontinuous motion as the load is transferred from the glide belt to the wheel.
After the wheels of the Lake hand truck have cleared the next step, the ram is retracted. There is no provision in the Lake hand truck to prevent the foot of the ram from engaging the nosing of the step during retraction, thereby damaging it. Likewise, there is no provision to prevent the ram from striking the edge of the step as it rotates about its foot as the truck is tilted and lowered onto the next step. This can result in damage to the ram as well as the step.
In Lake's hand truck, the pneumatic cylinder is pivotable with respect to the frame of the hand truck in a plane perpendicular to the axis of the wheels. As the wheels clear the next step, the cylinder must pivot through an angle with respect to the frame. When this angle exceeds a predetermined limit, an electrical limit switch is actuated, which causes the ram to be automatically retracted. This automatic retracting feature can be actuated at any point in the lifting phase if the operator inadvertently permits the top of the frame of the hand cart to move too far forward. This can be dangerous if it occurs before the wheels have gained the next step, because the sudden change in direction, being unexpected, throws the operator off balance. In addition, in the Lake hand truck, as before mentioned, the total excursion of the cylinder during its pivoting is limited to a very small value because of the presence of the common axle of the hand truck, thus further contributing to critical operation of the hand truck.